Chemical abundances of Seyfert 2 AGNs I. Comparing oxygen abundances from distinct methods using SDSS

TL;DR

This study compares various methods for estimating oxygen abundance in Seyfert 2 AGNs using SDSS data, revealing significant discrepancies among methods and confirming the Te-method's tendency to underestimate metallicity.

Contribution

It systematically evaluates different oxygen abundance estimation methods for Seyfert 2 AGNs, highlighting their discrepancies and limitations.

Findings

Strong-line methods differ by up to 0.8 dex in estimates.

The Te-method underestimates oxygen abundance in NLRs.

No correlation between host galaxy mass and AGN metallicity.

Abstract

We compare the oxygen abundance (O/H) of the Narrow Line Regions (NLRs) of Seyfert 2 AGNs obtained through strong-line methods and from direct measurements of the electron temperature (Te-method). The aim of this study is to explore the effects of the use of distinct methods on the range of metallicity and on the mass-metallicity relation of AGNs at low redshifts (z < 0.4). We used the Sloan Digital Sky Survey (SDSS) and NASA/IPAC Extragalactic Database (NED) to selected optical (3000 < A < 7000) emission line intensities of 463 confirmed Seyfert 2 AGNs. The oxygen abundance of the NLRs were estimated using the theoretical Storchi-Bergmann et al. calibrations, the semi-empirical N2O2 calibration, the bayesian Hii-Chi-mistry code and the Te-method. We found that the oxygen abundance estimations via the strong-line methods differ from each other up to ~0.8 dex, with the largest discrepancies in the low metallicity regime (12 + log(O/H) . 8.5). We confirmed that the Te-method underestimates the oxygen abundance in NLRs, producing unreal subsolar values. We did not find any correlation between the stellar mass of the host galaxies and the metallicity of their AGNs. This result is independent of the method used to estimate Z.